1. Field of the Invention
The invention relates to the field of integrated circuit fabrication and particularly to the formation of openings for ohmic contacts, and the like, through silicon dioxide layers.
2. Prior Art
During the fabrication of metal-oxide-semiconductor (MOS) integrated circuits, a glass layer (phosphorus doped SiO2) is often formed over an underlying thermally grown oxide (SiO2) layer. In the "rear end" processing of such devices, openings are made through the layers to expose underlying substrate regions. Then contact members such as aluminum contacts are formed in the opening in contact with the exposed substrate regions.
If a wet etchant is used to form the openings for the contacts, the openings become too large for high density circuits. This occurs since the phosphorus glass etches much faster than the thermally grown oxide. During the time that the etchant is attacking the thermally grown oxide, substantially more of the phosphorus glass is etched away, leaving a large opening.
If a plasma etchant is used to etch through both layers, relatively straight edged openings are formed in alignment with the overlying openings in the photoresist layer. The sharp corners of these openings are not particularly suitable for metal contacts since the metal layer coverage over these corners and into the contact openings is thinned, resulting in high current densities in the metal. In some cases in the prior art, a high temperature glass reflow step is used to round-off the sharp edges.
For the most part in the prior art, a compromise between the above-described wet etchant and plasma etchant is used. A dry plasma etchant is used to etch through the phosphorus glass followed by a wet etchant to etch through the thermally grown oxide. During the etching of the thermally grown oxide, the wet etchant enlarges the openings through the phosphorus glass thereby solving the problems associated with the sharp corners. A problem with this process is that the openings through the thermally grown oxide are relatively large, and more importantly, it is difficult to control the size of these openings. This, of course, means the underlying substrate regions (to which contact is being made) must be larger.
As will be seen, the present invention provides a tapered openings through the phosphorus doped glass and a narrow opening through the thermally grown oxide.
In another embodiment where a low phosphorus glass is used, the same result is achieved.